Water-absorbent resins have been widely used as absorbent articles for hygienic materials such as paper diapers and sanitary napkins, utilizing the feature that a water-based liquid, for instance, a body fluid such as human urine, blood or sweat is quickly absorbed in a large amount, so that the liquid once absorbed is not released even under a load.
Conventional water-absorbent resins have some problems that the water-absorbent resins discolor into yellow even immediately after the preparation depending upon the preparation processes, or that the resins are likely to discolor into yellow, brown or the like by an external factor such as heat or humidity when the resins are allowed to stand even if the resins are white immediately after the preparation. In the field of the above-mentioned hygienic materials such as paper diapers, when a water-absorbent resin discolors in the absorbent article, the commercial value as an absorbent article is drastically lowered. Therefore, under severe high-temperature, high-humidity environment such as storehouse in the summer time, it has been desired that a water-absorbent resin does not discolor when the water-absorbent resin or absorbent article is subjected to storage for a long period of time.
As water-absorbent resins having an effect of preventing discoloration, there have been known, for instance, a high water-absorbent polymer composition in which an organic phosphoric acid compound or a salt thereof is added to a high water-absorbent polymer (Japanese Patent Laid-Open No. Hei 5-86251); an absorbent composition comprising an acidic water-swellable crosslinked polymer, a basic water-swellable crosslinked polymer, a discoloration preventive and/or an antioxidant and/or a boron-containing compound (Japanese Patent Laid-Open No. 2000-230129); an absorbent composition comprising a water-absorbent resin, and an organic carboxylic acid and/or a salt thereof (Japanese Patent Laid-Open No. 2000-327926) and the like. However, the above compositions do not exhibit a sufficiently satisfactory effect of preventing discoloration when subjected to high-temperature, high-humidity storage for a long period of time.
Accordingly, an object of the present invention is to provide a water-absorbent resin having no discoloration immediately after the preparation, and having suppressed discoloration even when subjected to a room temperature storage or even to a high-temperature, high-humidity storage for a long period of time.
These and other objects of the present invention will be apparent from the following description.